Handle workspace/didChangeWatchedFiles

[renkun-ken]

There are some scenarios where some R files are created, changed, or removed outside editor operations which is not sent from client to server via lsp requests.

• Git checkout
• Rcpp/cpp11 generate R wrapper file for exported/registered C++ functions

In these cases, language server is not aware of the changes of the files, and thus will not update the workspace so that symbol definition and etc will not update accordingly until user explicitly open the changed files.

Therefore, we should handle workspace/didChangeWatchedFiles.

[renkun-ken]

For vscode client, it does not watch any files at the moment. We need to update the client options at https://github.com/REditorSupport/vscode-r-lsp/blob/master/src/extension.ts#L93 so that it watches all R files in the workspace.